The present invention relates to metering equipment and, more particularly, to electric watthour or kilowatthour meters.
Electric watthour or kilowatthour meters conventionally employ a metallic disk rotated by flux generated by opposed voltage and current stators disposed adjacent opposed surfaces of the disk against a magnetic drag. Polyphase kilowatthour meters measure the power and energy in two or more phases fed to a load by employing two or more sets of voltage and current stators angularly spaced about the disk.
In a kilowatthour meter having two sets of voltage and current stators, it is conventional to position a first of the sets on one side of the disk and the second of the sets on the other side of the disk.
It is conventional to perform several adjustments on the voltage and current stators, either in the factory or in the field, to calibrate the instrument. Two types of adjustment of interest to the present invention include phase lag adjustment and light-load adjustment on the voltage stator.
Conventional phase lag adjustment is performed at rated current and a power factor of 0.5 to adjust the disk rotational speed under these conditions. In one technique, a one-turn conductive loop of, for example, copper, encircles a substantial part of the flux passing between the core of the voltage stator and the disk. A phase lag plate built into the loop includes perforations joined by webs. A selected number of the webs are severed to adjust the ratio of reactance to resistance in the one-turn loop and thus to adjust the phase of the flux applied to the disk by the voltage stator with respect to the phase of the flux applied to the disk by the current stator.
The above conventional phase lag plate adjustment suffers from the incremental nature of the adjustment and the fact that, once severed, a web cannot be rejoined.
The incremental adjustment forces compromises between conflicting desires to obtain both a wide adjustment range and fine adjustment resolution. Such compromises usually result in a minimum adjustment step size of about 0.5 percent of disk speed. This limits the accuracy with which disk speed can be controlled in the presence of lagging power factor.
The irreversible nature of the adjustment not only makes field phase lag adjustment unavailable, but it also makes an adjustment error unforgiving. If one too many webs is severed, the kilowatthour meter must be withdrawn from production until a new phase lag plate is installed. Then the adjustment must be performed anew.
A vernier phase lag adjustment technique is disclosed in U.S. patent application Ser. No. 711,718 filed Mar. 14, 1985 now matured into U.S. Pat. No. 4,649,337 in which the variable resistance of the phase lag plate is replaced with a variable inductance consisting of a small number of turns of a copper coil closely wrapped by a plate of magnetic iron. The ratio of resistance to inductance in the one-turn phase lag loop encircling the flux passing between the voltage stator pole and the disk is smoothly controlled by adjusting the penetration of a steel adjustment screw along the axis of the copper coil.
As seen from the front of a kilowatthour meter, the voltage stator located at the left of the kilowatthour meter is rotated 180 degrees from the voltage stator located at the right of the kilowatthour meter. The standards of the American National Standards Association (ANSI), which govern kilowatthour metering equipment, require that adjustments should be accessible from the front of the kilowatthour meter and that a clockwise control adjustment on the voltage stator located at the left side of the kilowatthour meter should have the same effect on disk speed as a clockwise adjustment of a corresponding control on the voltage stator located at the right side of the kilowatthour meter. In the case of the referenced phase lag-adjustment device described in the preceding paragraphs, corresponding threaded holes are disposed at each end of the adjustment coil for permitting the adjustment screw to be inserted with its head facing in either direction. Thus, by selecting the end of the adjustment coil into which the adjustment screw is inserted, the same voltage stator may have its phase lag adjustment performed in the same direction, with the same direction of rotation of the adjustment screw, regardless of whether it is located on the left or the right side of the kilowatthour meter. That is, since advancing the adjustment screw into the adjustment coil increases the inductance in the phase lag loop, a clockwise adjustment of the adjustment screw has the same effect on disk speed in each location, as required by the ANSI standard.
A light-load adjustment on a voltage stator conventionally consists of a loop of magnetic or conductive material disposed between the pole of tha voltage stator and the disk. The loop is adjusted in a direction parallel to the disk and normal to a radius of the disk to adjust the disk speed at 10 percent rated current and unity power factor. The loop is conventionally moved by an adjustment screw. In order to provide the same adjustment effect in either left or right locations, it is conventional to use two different voltage stators: one with a light-load adjustment employing an adjustment screw with a right-hand thread; and the other with a light-load adjustment employing an adjustment screw with a left-hand thread. This permits locating voltage stators in left and right positions about the disk while providing the same rotational adjustment direction for the same adjustment of disk speed.
Different voltage stators, each requiring different parts which must be designed, manufactured and stocked, represent a significant increase in initial and continuing cost to both the manufacturer and the utility customer. It is thus desirable to avoid doubling the number of different voltage stator designs required to permit identical adjustment of left and right light-load adjustments. One approach to this objective is disclosed in U.S. patent application Ser. No. 590,973 filed Mar. 19, 1984, of common assignee with the present invention, which details a screw-type light-load adjust permitting adjustment from either side of the disk by merely changing the end of the light-load adjust into which the adjustment screw is inserted.
This referenced patent application, besides being of significantly different structure from the present application, lacks the additional integration of both light-load and phase lag adjustments, both of which may selectably be performed with the voltage stator installed in either the left or right positions, as viewed from the front of the kilowatthour meter.